1. Field
Exemplary embodiments relate to a semiconductor laser diode and a method for fabricating the same.
2. Description of the Related Art
Generally, semiconductor light emitting diodes are fabricated as chips that include a first semiconductor layer (e.g., an n-type GaN semiconductor layer) on a substrate; a quantum well-structured active layer on the first semiconductor layer; a second semiconductor layer (e.g., a p-type GaN semiconductor layer) on the active layer; a p-type electrode on an upper side of the first semiconductor layer; and an n-type electrode on an upper side of a portion of the second semiconductor layer, which is exposed after the first semiconductor layer and the active layer are partially etched.
Semiconductors based on group III nitrides represented by GaN are generally fabricated as device structures on a c-plane (0001) using a c-plane (0001) substrate (for example, a sapphire substrate). In this case, spontaneous polarization occurs in a c-plane (0001) direction that corresponds to a growth direction. In particular, when an LED having a representative InGaN/GaN quantum well structure is grown on a c-plane (0001), since a quantum-confined Stark effect (QCSE) is caused by piezoelectric fields due to an internal strain, caused by a lattice mismatch and the like, in the quantum well structure, there is a limit in improving the internal quantum efficiency of the LED.
Furthermore, group III nitrides, particularly GaN and alloys thereof (for example, alloys which include InN and/or AlN), are stable in a hexagonal wurtzite crystal structure defined by 2 or 3 equal basal plane axes at an angle of 120° between one another and perpendicular to a c-axis. Due to atomic positions of a group III element and nitrogen, any plane that is perpendicular to the c-axis contains only one type of atoms. Each plane along the c-axis can contain only one type of atom (i.e., group III element or nitrogen). Accordingly, in order to maintain charge neutrality, for example, in a GaN crystal, an N-face that contains only a nitrogen atom and a Ga-face that contains only a Ga atom are respectively located at ends of the GaN crystal. As a result, a group III nitride crystal exhibits polarity along the c-axis. Such spontaneous polarization is a bulk property and depends on the structure and composition of a crystal. Due to these characteristics, most of the GaN-based devices are grown in a direction that is parallel to the polar c-axis. In addition, when a hetero-structure is formed, since stress is generated due to a large difference in a lattice constant between group III nitrides and the same c-axis orientation of the group III nitrides, piezoelectric polarization is caused.
As such, in a general c-plane quantum well structure formed in group III nitride-based optoelectronic and electronic devices, an electrostatic field induced by piezoelectric polarization and spontaneous polarization changes an energy band structure of the quantum well structure, thereby distorting the distribution of electrons and holes. Such spatial separation of electrons and holes due to the electric field is referred to as the quantum-confined Stark effect. The quantum-confined Stark effect deteriorates the internal quantum efficiency and causes red shift of an emission spectrum and the like.
In addition, when a group III nitride-based material is used to form a semiconductor laser diode, problems such as band bending, non-uniform composition of a high indium-content active layer, reduction in the quantum efficiency of the laser diode at high electric potential density, and the like occur. These problems become more significant when a semiconductor laser diode for generating light with a long wavelength of 500 nm or more is realized. Although a non-polar substrate has been used to try to solve these problems, the non-polar substrate is more expensive than the c-plane substrate and has severe size constraints. Further, although attempts have been made to grow non-polar or semi-polar crystals on an m-plane, a-plane, or r-plane sapphire substrate, these attempts have not sufficiently resolved the above-described problems.